Papers by Francesco Rocca
Journal of Physics: Conference Series, 2013
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Journal of Physics: Conference Series, 2016
We propose an approach beyond the quasiharmonic approximation for interpretation of EXAFS and XRD... more We propose an approach beyond the quasiharmonic approximation for interpretation of EXAFS and XRD data and for ab initio calculations of electronic and vibration properties of materials with negative thermal expansion. Ab initio electronic structure and lattice dynamics calculations for cubic and distorted ScF3 were performed using the linear combination of atomic orbitals (LCAO) method. The band gap obtained in calculations for ScF3 is equal to 10.54 eV and agree well with the expected value. The calculated infrared spectra of F displaced (FD) cubic ScF3 allow us to predict that its mean Sc-F-Sc angle within NTE deviates from 180 degree.
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Materials Science and Engineering: A, 2009
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physica status solidi (c), 2004
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Physics and Chemistry of Glasses, 2000
The characterisation of the local structure of disordered materials is one of the major applicati... more The characterisation of the local structure of disordered materials is one of the major applications of x-ray absorption fine structure (XAFS) spectroscopy. XAFS site sensitivity has been widely exploited to describe the radial distribution around specific types of atoms in multicomponent systems such as doped borate glasses. Gaussian or near Gaussian distributions can be directly determined from XAFS measurements using the ratio method. However, the correct determination of the short range order in amorphous systems with asymmetric distributions requires the use of more refined data analysis procedures. In cases of moderate disorder the method based on the cumulant expansion of the XAFS signal allows XAFS to determine unbiased single shell radial distribution functions (RDF) without assuming a particular RDF model. XAFS temperature dependent measurements allow the separation of the static contribution to disorder from the thermal one; in such a way local vibrational dynamics parame...
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Journal of Nanoscience and Nanotechnology, 2011
ABSTRACT Atomic layer deposition (ALD) of nickel oxide was applied to the nickel-induced crystall... more ABSTRACT Atomic layer deposition (ALD) of nickel oxide was applied to the nickel-induced crystallization of amorphous Si thin films. The nickel-induced crystallization was monitored as a function of annealing temperature and time using Raman spectroscopy. Since Raman spectroscopy allows for the numerical quantification of structural components, the incubation time and the crystallization rates were estimated as functions of the annealing temperature. The spatial locations of a nickel-based species, probably NiSi2, were investigated using X-ray photoelectron spectrometry. The formed NiSi2 seeds appeared to accelerate the crystallization kinetics in amorphous Si thin films deposited onto glass substrates. The ramifications of the atomic layer deposition are discussed with regard to large-panel displays, with special emphasis on the sophisticated control of the catalytic elements, especially nickel.
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Optical Materials, 2006
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Physics and Chemistry of Glasses, 2006
The potential of XANES spectroscopy for studying the structure of boron-containing compounds is a... more The potential of XANES spectroscopy for studying the structure of boron-containing compounds is assessed by theoretical modelling ofB K edge spectra of minerals. Spectra arising from three-fold coordinated boron sites differ considerably from spectra arising from four-fold coordinated sites. The characteristic shape of the spectra is governed first of all by short range order and, to a lesser extent, by medium range order. Areas of main spectral peaks depend only on the coordination of the photoabsorbing site and can thus be used for quantifying the portion of boron atoms in BO 3 and in BO 4 units, in minerals as well as in glasses. The width of the main peak of spectrum at a four-fold coordinated boron site generally increases with the the spread of the B-O distances but deviations from this correlation may be substantial. In contrast to spectra of minerals, XANES of BPO 4 crystal is governed by long range order and is thus not a suitable model for studying XANES of borate glasses.
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Journal of Physics: Conference Series
Strontium titanate is a model quantum paraelectric in which, in the region of dominating quantum ... more Strontium titanate is a model quantum paraelectric in which, in the region of dominating quantum statistics, the ferroelectric instability is inhibited due to nearly complete compensation of the harmonic contribution into ferroelectric soft mode frequency by the zero- point motion contribution. The enhancement of atomic masses by the substitution of 16 O with 18O decreases the zero-point atomic motion, and low-T ferroelectricity in SrTi18O3 is realized. In this study we report on the local structure of Ti in SrTi16O3 and SrTi18O3 investigated by Extended X-ray Absorption Fine Structure measurements in the temperature range 6 - 300 K.
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Ferroelectrics
Strontium titanate is a model quantum paraelectric in which in the region of dominating quantum s... more Strontium titanate is a model quantum paraelectric in which in the region of dominating quantum statistics the ferroelectric instability is inhibited due to nearly complete compensation of the harmonic contribution into ferroelectric soft mode frequency by the zero-point motion contribution. The enhancement of atomic masses by the substitution of 18O for 16O decreases the zero-point atomic motion and low-T ferroelectricity in SrTi18O3 is realized. In this study we report on the local structure of Ti in SrTi16O3 and SrTi18O3 by Ti K-edge extended x-ray absorption fine structure measurements in temperature range 6 – 300 K.
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Physical Review B, 2016
Scandium fluoride (ScF3), having cubic ReO3-type structure, has attracted much scientific attenti... more Scandium fluoride (ScF3), having cubic ReO3-type structure, has attracted much scientific attention due to its rather strong negative thermal expansion (NTE) in the broad temperature range from 10 to 1100K.Herewe use the results of diffraction and extended x-ray absorption fine-structure (EXAFS) spectroscopy to interpret the influence of NTE on the temperature dependence of infrared absorption spectra of ScF3. Original infrared absorption and EXAFS experiments in a large temperature range are presented and interpreted using ab initio lattice dynamics simulations within and beyond quasiharmonic approximations. We demonstrate that ab initio electronic structure calculations, based on the linear combination of atomic orbitals method with hybrid functionals, are able to reproduce well the experimental values of lattice parameter a0, band gap Eg , and lattice dynamics in ScF3. However, the simulations performed within quasiharmonic approximation fail to reproduce the temperature dependence of two infrared active bands due to the F–Sc–F bending (at 220 cm−1) and Sc–F stretching (at 520 cm−1) modes present in the infrared absorption spectra. To overcome this problem, an approach beyond the quasiharmonic approximation is proposed: It accounts for the negative thermal expansion of the lattice and for fluorine atom displacements due to strong F vibrational motion perpendicular to the cubic axes and allows us to explain qualitatively the temperature behavior of infrared spectra of ScF3.
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Acta Physica Polonica A, 1991
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Acta Physica Polonica Series a
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The local coordination around Ag cations in silver borate glasses has been studied by comparing t... more The local coordination around Ag cations in silver borate glasses has been studied by comparing their experimental x-ray absorption near edge spectra (XANES) at the Ag K edge with results of theoretical simulations. Different trial structural models have been tested using known crystallographic environments as starting points for further modifications. The effort has been concentrated on exploring which structural features and trends improve the agreement between simulated spectra and experiment. The XANES of borate glasses cannot be reproduced by any of the crystalline sites. In order to describe the general shape of the spectrum, about eight or more neighbouring atoms within a radial distance of ~3·0 A have to be included. The most plausible model is characterised by a first shell Ag–O bond distance of R=2·2–2·3 A, distributing randomly the remaining atoms at longer distances. The substitution of some next nearest neighbour oxygen atoms with boron improves the agreement with exper...
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AIP Conference Proceedings, 2007
ABSTRACT Ziegler, E. de Panfilis, S. Peverini, L. van Vaerenbergh, P. Rocca, F.
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AIP Conference Proceedings
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MRS Proceedings
The local environment of light emitting silicon nanocrystals (Si-nc) embedded in amorphous SiO2 h... more The local environment of light emitting silicon nanocrystals (Si-nc) embedded in amorphous SiO2 has been studied by x-ray absorption spectroscopy (XAS) and by ab-initio total energy calculations. Si-nc have been formed by PECVD deposition of SiOx with different Si content (from 35 to 42 at.%) and thermal annealing at high temperature (1250 °C). The comparison between total electron yield (TEY) and photoluminescence yield (PLY) spectra has allowed the identification of a modified region of SiO2 (about 1 nm thick) surrounding the Si-nc, which participates to the light emission of Si-nc. Total energy calculations, within the density functional theory, clearly show that Si-nc are surrounded by a cap-shell of stressed SiO2 with a thickness of about 1 nm. The optoelectronic properties show the appearance of localized states not only in the Si-nc core region but also in the modified SiO2 region.
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Physical Review B
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Le Journal de Physique Colloques
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Le Journal de Physique Colloques
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Papers by Francesco Rocca